If $5a + 9b = -7$ and $9x + 5y + z = -8$, what is $90x + 10z - 90b - 50a + 50y$ ?
Solution: $= -50a - 90b + 90x + 50y + 10z$ $= (-10) \cdot (5a + 9b) + (10) \cdot (9x + 5y + z)$ $= (-10) \cdot (-7) + (10) \cdot (-8)$ $= 70 - 80$ $= -10$